The present application claims priority to Taiwan application No. 090213599 entitled xe2x80x9cWave receiving apparatus with parallel feeding elementsxe2x80x9d filed on Aug. 9, 2001.
1. Field of the Invention
The present invention relates to satellite communication technology. More particularly it relates to a wave feed structure for use in conjunction with an antenna dish for receiving satellite signals from space.
2. Description of the Related Art
Satellite communication is gaining importance in this world of real-time digital distribution of audio and video data around the globe. It is known that for the purpose of increasing the data capacity of a satellite system, for example a direct broadcast system (DBS), the technique of giving polarizations to data-carrying waves is commonly utilized. Polarization of an electromagnetic wave refers to the direction of the time-varying electric intensity field vector of the wave traveling in space. A linearly polarized (LP) wave is one whose electric intensity field vector points to a fixed direction, and a circularly or elliptically polarized (CP or EP) wave is one whose electric intensity field vector rotates periodically. Just as a LP wave can be decomposed into horizontal and vertical components in space quadrature, a traveling wave with circular or elliptic polarization can be constructed by superposition of two LP waves in space and time quadrature, that is, a horizontally polarized (HP) wave and a vertically polarized (VP) wave of 90-degree phase difference. In a typical satellite communication system, an antenna in the form of a reflector or dish with particular surface curvature is utilized to focus polarized waves collected from space into a signal feed device, such as a LNBF (Low Noise Block with integrated Feed) module, located in the focal point of the reflector surface. Since the reflector/LNBF assembly is a ground receiver with spatially fixed reception pins for detecting electric fields of waves transmitted from an orbiting satellite, when receiving CP waves characterized by rotating electric fields a device known as polarizer is required to convert CP waves into LP waves with spatially fixed electric fields for easy reception and vice versa.
FIG. 1, FIG. 2, and FIG. 3 illustrate the structure and construction of prior art LNBFs with polarizers. In FIG. 1a, LNBF 100 includes a waveguide 110 having a horn opening at one end for receiving polarized waves reflected from an antenna dish which convey audio and video signals in satellite communication. The received waves are guided afterward along the hollow conduit there within. A LNB circuits unit 120 disposed near the sidewall of the waveguide 110 is responsible for adapting the received audio and video signals for output to a TV set or other user device. In the example of FIG. 1b, LNBF 150 also includes a waveguide 160 for guiding received waves and a LNB circuits unit 170 for handling the signals contained in the received waves, but the LNB circuits unit 170 is mounted at the rear end of the waveguide 160. The LNBF 100 is shorter in overall length compared to the LNBF 150, and projects a smaller frontal area from a perspective looking into the horn opening. This is advantageous because when two LNBFs are required for an antenna dish capable of simultaneously receiving signals of two satellites, a LNBF with small frontal area allows itself to be more closely bundled with the adjacent one to reduce the lateral distance of the wave-receiving horns so that both can be more closely positioned at the focal point of the antenna dish, thereby upgrading their performance. It should be observed that the relative position between the waveguide and signal handling circuits in a LNBF module is subjected to system design choices.
FIG. 2a illustrates a polarizer 200 in the shape of two conducting plates set diametrically on the inner wall of the waveguide 110. The physical effect of the polarizer 200 is to alter the cross-sectional area of waveguide 110 in such a way that one component of an incoming CP wave shifts phase relative to the other component in time quadrature, and the CP wave is converted into two in-phase LP components when the phase shift between them reaches 90 degrees. Another example for producing phase shift in polarized waves is illustrated in FIG. 2b, wherein a dielectric slab 210 is added to the conducting waveguide 110 which alters due to changes in dielectric constant the phase velocity of one component of the received CP wave relative to the other to effect the CP/LP conversion.
FIG. 3 is a cross-sectional view of the waveguide 110, showing the polarizer 200 diagonally placed therein and a pair of signal collector pins 310, 320, one horizontal and the other vertical, protruding from the LNB circuits unit 120 into the hollow conduit thereof for collecting signals induced by the electric fields of polarized waves guided there within. Theoretically the conductor polarizer 200, and similarly the dielectric polarizer 210, is capable of converting the incoming CP wave into a LP wave that is to be received by the signal collector pins 310, 320. But in practice the conversion may be incomplete due to an imperfect polarizer or polarization distortions found in the received waves after traveling through the impure medium of atmosphere, so that signal collector pins may experience signal interferences when placed too close to each other. To avoid incomplete conversion, or cross-polarization, and to attain better pin-to-pin isolation, conventional design therefore places the signal collector pins a distance apart from each other along the axis of the waveguide as illustrated in FIG. 4. Usually a separating distance of half wavelength of the received wave is required for acceptable performance. Yet the distancing of the signal collector pins extends the overall length of the waveguide and hence a structurally bulky LNBF is formed.
In addition to the shortcomings of incomplete conversion of polarization and extended structure, conventional LNBF is disadvantageous in that, as shown in FIG. 3 and FIG. 4, the L-shaped collector pin 310 protruding form the LNB circuits unit 120 can not be easily and precisely positioned because it is not straight and conventionally Teflon materials are used to wrap around it which might produce gaps that make pin displacement and rotation possible, thereby causing inaccurate signal reception. Conventional LNBF is also disadvantageous in its manufacture processes. In the case of FIG. 2a, the waveguide 110 and the conducting polarizer 200 cannot be integrally formed as one piece by casting due to the shape of the polarizer 200. That is, the closed end portion of the waveguide 110 needs to be fixed to the rest after the cylindrical portion of the waveguide 110 and the conducting polarizer 200 are fabricated. Similarly in the case of FIG. 2b, additional step of bonding or gluing the dielectric polarizer 210 to the inner wall of the cylindrical portion of the waveguide 110 is necessary after the waveguide 110 is molded. In both cases, the waveguide/polarizer structure requires extra manual labor in its production.
The object of the present invention is to overcome the shortcomings of conventional LNBF described in the last section. The present invention consists of a conduit for guiding waves, having an open end allowing entrance of polarized waves reflected by the reflector; a septum polarizer monolithically formed with the conduit for effecting a circular-linear polarization conversion; a pair of signal collectors pointing to the same direction or towards each other and positioned at a distance of quarter-wavelength away from the rear end of the conduit for receiving wave signals; and a circuitry module, positioned sidelong next to said conduit seen from said open end into said conduit to which the signal collectors are electrically connected for handling wave signals.
Under such construction, the manual labor in the manufacture process is reduced by monolithically forming the septum polarizer with the conduit. The frontal area of the wave receiving apparatus is minimized by placing the circuitry module on the side instead of on the back of the wave-guiding conduit. Pin-to-pin isolation is improved by using the septum polarizer that thoroughly divides the conduit. And overall length of the conduit decreases, as the signal collectors are distanced less than half wavelength apart.